FIELD OF THE INVENTION
The invention lies in the field of optical transmission technology. Specifically, the invention relates to a configuration for coupling light into one end of a multimode optical waveguide by means of a pin stub from a single mode optical waveguide. On the input side, there is an optical device that focuses the light to be coupled onto one face end of the pin stub, and the pin stub rests with its other face end on the one end of the multimode optical waveguide.
A prior art system of this type forms a coupling unit, with which, as in a conventional plug-to-plug coupling, an optical transmission line formed by an optical waveguide can be connected via a plug. The prior art configuration includes an optical device on the input side which is acted upon by the light to be coupled into the optical waveguide. The light to be coupled in is focussed by the optical device onto one face end of a pin stub from a single mode optical waveguide. With its other face end, the pin stub contacts one end of the optical waveguide that forms the optical transmission line. The prior art arrangement was developed for coupling light into a single mode optical waveguide, but it can readily be used to couple light into a multimode optical waveguide as well.
Multimode optical waveguides have been laid out extensively, especially over relatively short distances for transmitting information in the area of data communications, because relatively inexpensive data transmission systems can be made with such optical waveguides. A disadvantage of such data transmission systems having multimode optical waveguides is the bandwidth, which is dependent on the wavelength of the light employed and on the particular type of multimode optical waveguide; this bandwidth is relatively narrow compared with single mode optical waveguides. The bandwidth of data transmission systems with multimode optical waveguides is also negatively influenced by the type of light source used and by the coupling of the light, which carries the information in question, into the multimode optical waveguide. If a light-emitting diode (LED) is used as the light source, then a multimode optical waveguide is generally illuminated over virtually its entire face end to be coupled in, and thus virtually all the modes are excited. Because of the differences in transit times among the various modes, only a relatively narrow bandwidth is attainable. If a laser diode is used, a smaller number of modes in the multimode optical waveguide is excited, especially whenever a focusing optical device is also used between the laser diode and the multimode optical waveguide.
It is especially advantageous if, in addition to the focusing optical device, a pin stub from a single mode optical waveguide is used, since the number of modes in the downstream multimode optical waveguide that are excited is reduced because of the spatially narrowly bounded core of the single mode optical waveguide of the pin stub. However, because of the relatively short pin stub, a high proportion of the light coupled into the pin stub appears in the cladding of the single mode optical waveguide of the pin stub. Since when the pin stub is connected to the downstream multimode optical waveguide the adjacent face ends of these two optical waveguides rest flush against one another, some of the light in the cladding of the pin stub is then also coupled into the core of the multimode optical waveguide and excites higher modes. This narrows the bandwidth of the multimode optical waveguide.